A conventional internal combustion engine includes a catalyst for purifying exhaust gases. In order to fulfill a function of the catalyst, an air-fuel ratio of exhaust gases is determined, and a volume of fuel to be injected to the internal combustion engine is controlled so that the air-fuel ratio becomes a predetermined air-fuel ratio. The air-fuel ratio is detected by an exhaust gas sensor provided at an upstream side of the catalyst.
When exhaust gases from multiple cylinders of the internal combustion engine are collected in an exhaust pipe and one exhaust gas sensor is provided in the exhaust gas pipe in which the exhaust gases are collected, the exhaust gases from each of the cylinders are not dispersed evenly within the exhaust gas pipe. Moreover, among the exhaust gases to be in contact with the exhaust gas sensor, exhaust gases from a specific cylinder have a fast flow rate, while exhaust gases from other cylinders have a slow flow rate. For this reason, a value detected by the exhaust gas sensor is different depending on each of the cylinders.
When the exhaust gas sensor is provided in each exhaust gas port in the multicylinder internal combustion engine with the multiple cylinders, many exhaust gas sensors are necessary. To solve this matter, the following method is known: as disclosed in Patent Document 1, exhaust-gas communication paths for guiding exhaust gases, respectively, from the exhaust gas ports are joined, together; and by providing the exhaust gas sensor at the joining point, an air-fuel ratio in the multiple cylinders is determined by a small number of the exhaust gas sensors.